Sun Java System Identity Manager (IdM) 7.0 through 8.0 responds differently to failed use of the Forgot Password feature depending on whether the user account exists, which allows remote attackers to enumerate valid usernames.
http://www.vupen.com/english/advisories/2009/0797
http://sunsolve.sun.com/search/document.do?assetkey=1-66-253267-1
http://sunsolve.sun.com/search/document.do?assetkey=1-21-140936-01-1
http://securitytracker.com/id?1021881
http://secunia.com/advisories/34380
http://blogs.sun.com/security/entry/sun_alert_253267_sun_java